As shown in a number of recent publications, nanostructures with a surface particle density on the order of 1012 cm−2 are promising for development of efficient nanoelectronic devices, such as ultrafast switches or subminiature memory cells (K. -H. Yoo, J. W. Park, J. Kim, K. S. Park, J. J. Lee and J. B. Choi. Appl. Phys. Lett., 1999, v. 74 (14), p. 2073).
For example, in the case of densely packed nanostructures with grain size of ˜4 nm, it is possible to create storage devices with recording density of ˜1011 bit/cm2 (F. Pikus and K. Likharev. Appl. Phys. Lett., 1997, v. 71, p. 3661; Y. Naveh and K. Likharev. Superlattices and Microstructures 2000, v. 27, p. 1). In the limiting case of grain size diminished to ˜1 nm, the recording density increases to 1012 bit/cm2.
In the last decade, a new area of catalytic chemistry has formed and is rapidly developing now: heterogeneous catalysis on nanostructured materials (P. S. Vorontsov, E. I. Grigor'ev, S. A. Zav'yalov, L. M. Zav'yalova, T. N. Rostovschikova, O. V. Zagorskaya, Himicheskaya Physica 2002, v. 21, p. 1). Most of the catalysts that are studied in laboratories and used in technological practice contain nanoparticles, i.e., particles with dimensions in the range 1–100 nm. The fundamental distinction between nanoparticles and bulk materials is that the fraction of surface atoms in nanoparticles is comparable with that in the bulk and the radius of curvature of the surface is comparable with the lattice constant. It is a commonly accepted opinion that it is these specific features that ensure the high catalytic activity of nanostructured catalysts as compared with their analogues based on bulk materials. The most promising for quite a number of practically important applications are catalysts based on metallic nanostructures, which contain nanoparticles of Cu, Pt, Pd, Ni, Fe, Co, and other metals.
The known methods for obtaining nanoparticles of various materials can be divided into two large groups: in the first of these, nanoparticles are formed by combination of atoms (or more complex radicals and molecules), and in the second, by dispersion of bulk materials.
Numerous methods based on combination of atoms (radicals, molecules) into nanoparticles are known, including, e.g., thermal evaporation and condensation (see S. Tohno, M. Itoh, S. Aono, H. Takano, J. Colloid Interface Sci., 1996, v. 180, p. 574), ion sputtering (see U.S. Pat. No. 5,879,827, Int. Cl. H 01 M 04/36, published Sept. 03, 1999), reduction from solutions (see U.S. Pat. No. 6,090,858; Int. Cl. C 09 K 03/00, published 18 Jul. 2000), and reduction in microemulsions (see H. Herrig, R. Hempelmann, Mater. Lett., 1996, v. 27, p. 287).
For example, in the method for obtaining nanoparticles by reduction of metals from solutions, an aqueous solution of a metal salt and an anion-active compound with COO−, SO42−, or SO32− groups as a reducing agent is heated to 50–140° C., with the result that the metal salt is reduced to give metallic nanoparticles (see U.S. Patent Application Ser. No. 20020194958; Int. Cl. B 22 F 09/24, published 26 Dec. 2002).
In the known method for deposition of submonolayer and monolayer coatings composed of gold and silver nanoparticles, the structure is formed via capture of metallic nanoparticles prepared in a colloid solution onto the support surface covered by a special organic film (see U.S. Pat. No. 6,090,858; Int. Cl. C 09 K 03/00, published 18 Jul. 2000).
The advantage of this method consists in that it enables immobilization on the support surface of spherical nanoparticles with average size in the range from 3 to 100 nm (depending on preparation conditions) with rather narrow size dispersion. However, the maximum surface density of particles on the support surface does not exceed in this case 0.5 d2 (where d is the average size of nanoparticles). Accordingly, exchange of electrons between neighboring particles is hardly probable and it is impossible to use structures of this kind to create catalysts operating in the maximum efficiency mode and to design efficient nanoelectronic devices in which the effects of interaction and charging of densely packed particles are important.
A method is known for obtaining silicon clusters in structural voids of zeolites, which consists in introduction of disilane (Si2H6) into these voids and its subsequent oxidation. Silicon liberated in the reaction assembles into nanoclusters. This technique is a particular case of the chemical vapor deposition (CVD) method (see Dad O., Kuperman A., MacDonald P. M., Ozin G. A.—A New Form of Luminescent Silicon—Synthesis of Silicon Nanoclusters in Zeolite-Y.—Zeolites and Related Microporous Materials: State of the Art., 1994, v. 84, p.p. 1107–1114). The method cannot be used to form silicon nanostructures in local regions because it transforms the zeolite substrate across virtually its whole thickness. An, in fact, homogeneous composite material is produced by this known technique.
Also known is a method of cryochemical synthesis of metal-polymer nanostructures (see L. I. Trakhtenbers et al., Zh. Fiz. Khim., 2000, vol. 74, p. 952).
The main advantage of metal-polymer nanostructures is their rather high specific activity as catalysts. However, as the content of metal increases, the catalytic activity of catalysts of this kind decreases because crystalline nanoparticles formed by this technique coagulate when coming in contact with one another. Moreover, the fundamental aspects of nanoparticle growth, inherent in the cryochemical synthesis, necessarily lead to a broad distribution of particle sizes and shapes.
To methods of the second group (formation of nanoparticles by dispersion of materials) should be referred the technique (see K. Deppert and L. Samuelson. Appl. Phys. Lett.—1996, v. 68(10), p. 1409) in which the initial flow of polydisperse liquid drops is produced in the course of thermal evaporation of an overheated material, capture of drops by the flow of an inert carrier gas (nitrogen), and, further, successive separation of particles via interaction of charged particles in the gas flow with the electric field in the differential mobility analyzer. The thus formed flux of charged nanoparticles is deposited onto the substrate. This method, named “Aero taxi” by its authors, makes it possible to obtain a monodisperse flux of charged nanosize particles of metals (and semiconductors). The method yields 20–30-nm crystalline particles, with the particle size dispersion not less than 50% (the size scatter directly depends on the number of separation stages). Among disadvantages of the method are its low output capacity and relatively wide particle size dispersion. Moreover, the method gives no way of forming metallic particles with high density of particle packing, because, as the density increases, crystalline nanoparticles coagulate into more bulky formations.
The method for obtaining nanoparticles, which is the closest to that claimed in this patent application and is chosen as the prototype, was described in (V. M. Kozhevin, D. A Yavsin, V. M. Kouznetsov, V. M. Busov, V. M. Mikushkin, S. Yu. Nikonov, S. A. Gurevich, and A. Kolobov, J. Vac. Sci. Techn. B, 2000, v. 18, no. 3, p. 1402). This method is based on ablation of a metallic target under the action of light generated by a high-power pulse-periodic laser. Rather severe modes of target irradiation are chosen, in which, together with evaporation of the target, a great number of micrometer- and submicrometer-size drops of molten metal are ejected from its surface. Optical breakdown of a vapor near the target surface leads to the formation of a hot laser torch plasma, while the temperature and density of this plasma are determined by the type of a metal and conditions of target irradiation (power density of the incident laser light, angle of incidence, etc.). In the laser torch plasma, liquid metal drops ejected from the target surface are charged to a critical value, to the threshold of capillary instability, on reaching which drops start to break down to produce a multitude of finer (daughter) drops. The daughter drops are charged to above the instability threshold, so that the breakdown process that has started is of a cascade nature. However, it was shown in the publication mentioned above that the process of drop breakdown continues only to a certain extent. This process terminates because, as the size of charged drops steadily decreases, the current of autoelectronic emission from their surface grows, which, in the end, leads to a decrease in the drop charge to below the instability threshold. For most of metals, the size of drops formed by the end of the breakdown process is on the order of several nanometers. The abrupt termination of the process ensures a sufficiently narrow size distribution of the resulting nanoparticles. Thus, the breakdown of liquid micrometer- and submicrometer-size metal drops in the laser torch plasma yields a great number of nanometer-size particles with a narrow size distribution.
The prototype method described has been used to deposit onto the substrate surface single-layer coatings composed of 8–10 nm copper nanoparticles. Even though the particle size dispersion was not evaluated for the prototype method, it may be concluded, on the basis of the results obtained in this study, that the size distribution is much narrower than, e.g., that for the “Aero taxi” method.
However, the conditions that could ensure stable formation of nanoparticles with amorphous structure have not been determined for the prototype method. This circumstance markedly restricts the possibility of reproducible formation of nanostructures with high surface density of particles on the support surface, which is very important, e.g., for carrying out effective catalysis and developing a number of nanoelectronic devices. This also hinders the industrial application of the prototype method.
It should also be noted that, as established by the authors, the range of plasma parameters in which an effective breakdown of liquid charged drops can be achieved is considerably wider. This makes it possible to produce nanoparticles not only in a laser-induced plasma, but also in a plasma formed by other, more technologically convenient methods yielding a quasi-stationary plasma.
In the known method for obtaining nanoparticles, the conditions are close to equilibrium, which leads to the formation of metallic particles that are, as a rule, in the crystalline state. The coalescence of crystalline nanoparticles gives rise to severe difficulties in formation of structures with a high density of particle packing.